CNN) — A single lethal injection was supposed to speedily end Joseph Wood’s life last week. When it didn’t, Arizona executioners gave him a total of 15 doses of a novel drug cocktail, according to prison documents released Friday.

It took nearly two hours for Wood to die, as he gulped and snarled for air, journalists attending the execution said.

Arizona’s attorney general has temporarily halted executions after the convicted murderer’s prolonged death. Republican Sen. John McCain has called it tantamount to torture.

Gov. Jan Brewer called for an internal investigation by the corrections department.

But on Friday, Democratic state Sen. Ed Ableser added his voice to those of critics calling the execution “botched” and saying that it is not enough for the corrections department, which carried it out, to investigate itself.

He repeated the call for an independent investigation.

Two lawyers not connected to the case and who looked at the execution protocol late Friday agreed.

Richard Dieter heads the Death Penalty Information Center. It looks into legal snares and discrimination surrounding the application of the death penalty but neither supports nor opposes it per se, Dieter said.

“An execution itself should be humane and constitutional,” he said.

This execution went wrong. It did not follow protocol, in his opinion. “Instead of applying that (single injection) and saying it didn’t work, they kept applying doses.”

The state used midazolam, an anesthetic, and hydromorphone, a narcotic painkiller which, with an overdose, halts breathing and stops the heart from beating. It’s one of the new combinations that states have tried, with some controversial results, after manufacturers based or operating in Europe prevented U.S. prisons from using their drugs in executions.

Instead of conceding failure with the relatively new drug mixture, they kept trying to kill Wood with repeated doses, Dieter said.

“The bungled execution of Joseph Wood is unprecedented and without compare,” she said. Wood was supposed to receive one dose only.

“The department violated its protocol,” she said after reviewing the corrections document.

Both lawyers said that the state should have made the protocol and drugs used public before the execution and given experts a chance to weigh in on their effectiveness.

Death penalty proponents have argued that doing so allows death penalty opponents to pick execution methods apart, possibly preventing them.

I’m greatly troubled,” U.S. Attorney General Eric Holder told PBS Newshour on Thursday. He has been tasked with a national review of the death penalty methods after botched executions involving midazolam.

Making the injection protocol public ahead of time is not legally required, he said. But his moral convictions were in line with the critics’. “It seems to me, just on a personal level, that transparency would be a good thing,” he said.

penalty in some cases, but he said Wood’s execution was carried out in a “terrible” way. “The lethal injection needs to be an indeed lethal injection and not the bollocks-upped situation that just prevailed. That’s torture,” he said.

In the immediate wake of the execution, the corrections department issued a statement saying that it had gone according to procedure.

Wood was deeply sedated the entire time, it said.

State officials and relatives of the convicted killer’s two victims disagreed that he had been gasping for air, saying Wood snored and didn’t appear to be in agony.

Reports that the execution was botched are “erroneous,” Corrections Director Charles Ryan said.

Jeanne Brown, a close relative of Wood’s victims, said he got what he deserved.

Wood was convicted of murder and assault in the 1989 deaths of his estranged girlfriend and her father.